Here's what I do, have them pick out their own. Paladins must choose 4, and then another every 5 levels. Clerics & monks 2 vows; anyone else may pick a free feat if they choose a vow.
Obedience: obey all leaders & those of higher level in their specific command
Loyalty: renounce all ties to family, guild & government. Live for their order.
Bravery: Never show fear. May retreat but not while defending.
Courtesy: show respect to those of higher level & station.
Challenge: You are open to a challenge in whatever form, at any time.
Humility: remain humble in spirit, rejecting adulation and reward.
Celibacy: remain chaste.
Neutrality: apolitical, do not get involved in politics
Minor Faith: Accept no aid from non-allied churches.
Major Faith: Accept no aid from another's alignment. (Counts as 2)
Truth: do not lie or misrepresent the truth.
Freedom: allow no intelligent being to remain in slavery
Minor Poverty: surrender 25% of all wealth attained.
Major Poverty: surrender 75% of all wealth attained. (Counts as 2)